This application claims the priority of Japanese Patent Application No. 2001-156653 filed on May 25, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a compact single-focus lens constituted by three elements and, in particular, to a single-focus lens suitable for digital cameras.
2. Description of the Prior Art
Along with the widespread use of personal computers, digital cameras which can easily process captured image information have been becoming pervasive in recent years.
Such digital cameras are required to be more compact and less expensive, which causes an urgent demand for their taking lenses to achieve compactness and low cost.
As taking lenses for fulfilling such a demand, those disclosed in Japanese Unexamined Patent Publication No. HEI 9-258100, Japanese Unexamined Patent Publication No. 2000-180719, and the like have been known.
However, each of the taking lenses disclosed in the above-mentioned publications uses at least four lenses, and thus is demanded to achieve further compactness and lower cost.
Meanwhile, digital cameras use small-sized electric imaging devices, thereby making it necessary for principal rays to be made incident on the whole screen substantially at right angles, whereas a higher aperture efficiency and a longer back focus are required, and so forth, whereby the idea of designing their taking lens is greatly different in principle from that in conventional compact cameras using silver halide films.
In view of circumstances mentioned above, it is an object of the present invention to provide, as a taking lens mounted to a compact camera, a digital camera in particular, a single-focus lens which can achieve compactness and lower cost while favorably correcting aberrations by using a three-lens configuration.
The single-focus lens in accordance with the present invention comprises, successively from an object side, a first lens with a low power having at least one aspheric surface, a second lens with a positive refracting power having a convex form on an image surface side, and a third lens with a low power having at least one aspheric surface.
Preferably, a stop is disposed between the first and second lenses.
Preferably, the first and second lenses are formed from a plastic material.
Preferably, the single-focus lens of the present invention satisfies the following conditional expressions (1) to (3):
|fxe2x80x2/f1xe2x80x2| less than 0.5xe2x80x83xe2x80x83(1) 
0.5 less than fxe2x80x2/f2xe2x80x2 less than 2.0xe2x80x83xe2x80x83(2) 
|fxe2x80x2/f3 less than 0.5xe2x80x83xe2x80x83(3) 
where
fxe2x80x2 is the focal length of the whole lens system;
f1xe2x80x2 is the focal length of the first lens;
f2xe2x80x2 is the focal length of the second lens; and
f3xe2x80x2 is the focal length of the third lens.
Preferably, the first lens has a meniscus form with a concave surface directed onto the object side.
Preferably, the single-focus lens of the present invention further satisfies the following conditional expressions (4) and (5):
1.65 greater than Nd2xe2x80x83xe2x80x83(4) 
50 less than xcexdd2xe2x80x83xe2x80x83(5) 
where
Nd2 is the refractive index of the second lens at d-line; and
xcexdd2 is the Abbe number of the second lens at d-line.
The second lens may have a substantially flat surface on the object side.
Each of the first and third lenses may have an aspheric surface on each side thereof.